A bacterial cell wall is a highly complex structure that encases the cell, provides it shape and maintains internal osmotic forces. The cell wall includes a thick layer of peptidoglycan, which is a heteropolysaccharide polymer comprised of subunits of alternating N-acetylglucosamine (Glc-NAc) and N-acetylmuramic (MurNAc) acid (also known as the glycan chain). The MurNAc moiety has a pentapeptide chain attached. Included subunits are linked by β1-4 glycosidic bonds and cross-linked via the peptide chains through alternating L- and D-amino acids. Various types of peptidoglycan are identified by the type of cross-linkage and the specific amino acid at the third position of the peptide chain. Bacillus anthracis has an A1γ type peptidoglycan, with the “A” indicating cross-linkage between positions 3 and 4 in the peptide chain; “1” indicating that it is a direct cross-linkage; and the “γ” (gamma) indicating that the direct cross-linkage is between meso diaminopimelic acid residue (DAP) and the D-Ala in position 4. Recent studies (such as Candela, T. et al.; N-Acetylglucosamine Deacetylases Module the Anchoring of the Gamma-Glutamyl Capsule to the Cell Wall of Bacillus Anthracis. Microbial Drug Resistance. 2014, 20(3), 222-230) indicate a high level (e.g., 92%) of peptidoglycan modification by N-deacylation of GlcNAc residues. This modification is typically responsible for the cell wall's resistance to lytic enzymes such as lysozyme and mutanolysin. Candela, T. et al., also reported that the cross-linkage of peptidoglycan in Bacillus anthracis is higher than previously reported.
Peptidoglycan hydrolases are a diverse group of enzymes that cleave the cell wall at specific structural sites and are responsible for the highly-regulated cleavage of peptidoglycan during cell growth and division. Peptidoglycan hydrolases can be organized into 4 classes: (i) amidases, (ii) endopeptidases, (iii) glucosaminidases, and (iv) lysozymes, each of which cleaves a specific bond within the peptidoglycan or its fragments. Peptidoglycan hydrolases typically comprise a substrate binding domain and an activity domain. The activity of a particular peptidoglycan hydrolase (lysin) may be dependent on one or both of these domains. Exogenous application of some lysin proteins to bacterial cells results in hydrolysis of peptidoglycan, and cell lysis due to osmotic shock.